Although several lines of evidence suggest that increased levels of O-GIcNAc modification interferes with normal glucose homeostasis, the exact functional consequences of this modification remain to determined. Recent data indicate that O-linked GIcNAc transferase (OGT1) in a complex with the co-repressors Hdac1 and mSin3A mediates repression of gene expression in the HepG2 liver cell line. However, the target genes and the conditions under which this repressor complex is active in vivo are not known. Insulin gene expression in the pancreatic beta cells is induced by high blood glucose levels and is decreased when blood glucose levels are low. Using the deacetylase inhibitor trichostatin A, we have recently discovered that insulin gene expression is inhibited at low concentrations of glucose by the recruitment of deacetylases to the insulin promoter in the mouse insulinoma (MIN6) cell line. Furthermore, we found that OGT1 and Hdac1/2 are recruited to the insulin gene promoter and interact with each other only at low levels of glucose in the MIN6 beta cells. Based on these data, we hypothesize that OGT1 and Hdac1/2 are in a complex and repress insulin gene expression at low levels of glucose. We will test this hypothesis as described in the following three specific aims. 1) To analyze the role of OGT1 in repression of insulin gene expression in MIN6 beta cells. 2) To identify the transcription factor(s) that recruits the OGT1-Hdac1/2 repressor complex to the insulin gene promoter. 3) To identify genes with altered transcription in response to increased O-GIcNAc modification. Understanding the role of OGT1 in regulating insulin gene transcription will provide new insights into the mechanisms by which defects in O-GIcNAc modification lead to symptoms associated with diabetes. In addition, the obtained data will be instrumental in the design of novel strategies for the treatment and prevention of type I and type II diabetes and the secondary complications associated with diabetes.